Radio frequency power amplifiers are widely used to transmit signals in communications systems. Typically a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information is sent in the form of modulation of amplitude, phase or frequency or some combination of these which causes the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes the carrier itself is not sent since it is not essential to the communication of the information.
When a signal, which contains amplitude variations, is amplified it will suffer distortion if the amplifier does not exhibit a linear amplitude transfer characteristic. This means that the output is not linearly proportional to the input. It will also suffer distortion if the phase shift which the amplifier introduces is not linear over the range of frequencies present in the signal, or if the phase shift caused by the amplifier varies with the amplitude of the input signal. The distortion introduced includes intermodulation of the components of the input signal. The products of the intermodulation appear within the bandwidth of the signal causing undesirable interference, as well as outside the bandwidth originally occupied by the signal. This can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements.
Although filtering can be used to remove the unwanted out of band distortion, this is not always practical, especially if the amplifier is required to operate on several different frequencies. Distortion products which are at multiples of the carrier frequency can also be produced in a nonlinear amplifier, but these can typically be removed by filtering.
Intermodulation is also a problem when multiple signals are amplified in the same amplifier even if individually they do not have amplitude variations. This is because the combination of the multiple signals produces amplitude variations as the various components beat with each other by adding and subtracting as their phase relationships change.
Amplifiers can introduce some distortion even if they are well designed. Perfect linearity over a wide range of amplitude is difficult to realize in practice. Moreover, as any amplifier nears its maximum output capacity, the output no longer increases as the input increases and thus it becomes nonlinear. A typical amplifier becomes significantly nonlinear at a small fraction of its maximum output capacity. This means that in order to maintain linearity the amplifier is often operated at an input and output amplitude which is low enough such that the signals to be amplified are in the part of its transfer characteristic which is substantially linear. This method of operation is described as "backed off," in which the amplifier has a low supplied power to transmitted power conversion efficiency. A "Class A" amplifier operated in this mode may be linear enough for transmitting a signal cleanly, but might typically be only 1% efficient. This wastes power and means that the amplifier has to be large and relatively expensive. It also means that the wasted power is dissipated as heat which has to be removed by a cooling system.
Communication schemes using signals which have constant amplitude with frequency and phase modulation can use highly nonlinear amplifiers. These types of signals are unaffected by the distortion and the amplifiers can be smaller, cooler, more power efficient and less expensive. Modulation of this type is used in conventional radio paging systems which use CPFSK modulation.
Many of the newer, bandwidth efficient modulation schemes have both amplitude and phase variations. There is also a desire to be able to transmit multiple signals on different channels through a single amplifier. This reduces the number of separate amplifiers required and avoids the need for large, costly high level output signal combining filters which have undesirable power losses.
In the prior art, linearized amplifiers can be made by correcting for the nonlinearities of amplifiers using mechanisms such as cartesian feedback, predistortion and feedforward correction.
Cartesian feedback is a mechanism in which a monitoring system looks at the output of the amplifier and attempts to alter the input of the amplifier so that it produces the intended output. This is arranged as a direct feedback loop. The delay in the feedback path means that the correction can be too late to correct effectively, especially at higher bandwidths.
The predistortion mechanism attempts to correct for the nonlinear transfer characteristic of an amplifier by forming an inverse model of its transfer characteristic. This characteristic is applied to the low level signal at the input of the amplifier in a nonlinear filter, to pre-distort it such that when it passes though the amplifier the signal emerges amplified and substantially undistorted. This method is capable of excellent results over a relatively small bandwidth. The filter has to be updated to account for variations in the amplifier transfer characteristic and this is done by monitoring the output and periodically updating the corrections. The filter also has to change its coefficients as often as every sample using the values stored in memory.
The feedforward mechanism derives a signal which represents the inverse of the distortions produced by the amplifier. This is done by comparing the amplifier input and output. A small linear amplifier is used to amplify the distortion signal. This signal is then subtracted from the main amplifier output. This method operates correctly over a wider bandwidth than the predistortion mechanism. However, balancing the amplitude and delay of the distortion signal so that it cancels the main amplifier errors exactly is complicated to perform.
Both predistortion and feedforward are widely used in commercial products which can amplify multiple signals and work over wide amplitude ranges. Both methods are quite complicated and the power efficiencies are still not excellent. Feedforward amplifiers are typically only 5% efficient. The complicated processing requirements add to the cost and the power used and significant cooling capacity is still required to remove waste heat.
Another prior art amplifier is the LINC (Linear Nonlinear Component) amplifier 10, as shown in FIG. 1. A signal which has amplitude variations can be generated by two signals which vary only in their relative phases. The vector sum of the two signals can represent any amplitude. Thus, it is possible to represent the instantaneous state of any signal or combination of signals. The phase and frequency of the component signals can also be made to represent that of the original so that when combined, the original signal is reconstructed.
In FIG. 1, LINC amplifier 10 amplifies two or more constant amplitude signals, which represent an input signal to be amplified. The LINC amplifier uses a signal separator 11 to split the input 12 into the two components 13, 14, which are constant amplitude, phase varying components. The LINC amplifier may be supplied a complex baseband digitally sampled signal 12. The baseband signals 12 can be a representation of multiple modulated carriers using any modulations. For simplicity, various details such as the need to convert from baseband to a higher frequency and the need to convert from digital into analog have been omitted.
Since amplitude variations do not have to be dealt with, it is possible to build an amplifier which will amplify signals linearly by using the two phase and frequency modulated components. The nonlinearity of the amplifiers is no longer a problem in the amplification of multiple signals or those containing amplitude variations because the constant amplitude of the two components 13, 14 become constant amplified amplitudes as they are amplified by amplifiers 15, 16, while the phase of the components passes through the amplifiers with a constant shift. Although the nonlinear amplifiers produce distortion signals at multiples of the carrier frequency, these can be filtered off.
A problem occurs when the LINC mechanism is used for radio communication transmission at radio frequency ("RF"). Prior art descriptions which refer to the LINC idea have principally described methods of generation of the two phase component signals from an input signal, as shown in FIG. 1. A very high degree of accuracy in the phases and amplitudes of the two components, 13, 14 is required in order to achieve proper operation. If the two components 13, 14 are not extremely well balanced, the distortions seen at the output of combiner 17 (which recombines the amplified signals of components 13 and 14) due to the effect of the imbalances can be worse than the effects of an amplifier non linearity. A typical prior art arrangement might only generate a signal which is 20 dB above its wideband intermodulation noise floor. This is not sufficient for most base station transmitter applications where 60 to 80 dB is often required.
Therefore, there is a need in the art for a modern radio communication system to have power amplifiers for multiple signals and signals which have varying amplitude. Moreover, there is a need for an amplifier unit which is power efficient and inexpensive. Current solutions to this problem are linearized amplifiers which are complicated and not particularly efficient. Prior art LINC amplifiers cannot be used because the two components cannot be accurately combined to the required degree of precision without the deleterious effects of imbalance.